1. Field of the Invention
The present invention relates to an image forming apparatus such as, mainly, an electrophotographic or electrostatic copying machine and printer, that can correct a positional deviation in an image. Further, the present invention relates to a positional deviation detection apparatus and a positional deviation detection method for detecting a positional deviation in an image.
2. Description of the Related Art
Currently, in image forming apparatuses for forming color images, there is a growing demand for improvement of stability of their image qualities. The color image is made of, for example, a yellow (Y) toner, a magenta (M) toner, a cyan (C) toner, and a black (Bk) toner. An arbitrary color image can be formed by superimposing toner images formed from the toners of the respective colors on one another. However, electrophotographic color image forming apparatuses may form the toner images of the respective colors with a deviation generated among relative positions thereof due to tolerances of rotational speeds of image bearing members that bear the toners of the plurality of colors, a temperature change in the apparatus, an error in relative positions of a driving member and an image forming member, a change over time, and the like. This is a state with a so-called color deviation generated in a color image. The deviation among the relative positions of the toner images of the respective colors also leads to generation of a deviation in the color image formed by superimposing these toner images on one another, thereby deteriorating an image quality of the color image.
Therefore, conventionally, a positional deviation correction has been periodically performed as a registration operation for correcting relative positional deviation amounts of the toner images of the respective colors. In the positional deviation correction, first, a positional deviation correction pattern is formed on a rotating member such as an intermediate transfer member. Then, a position of a patch of each color as the positional deviation correction pattern is detected by a sensor as a detection unit. More specifically, light is emitted from a light emitting element, and reflection light from the patch and the image bearing member is received by a light receiving element, by which the position of the positional deviation correction pattern is detected. Relative positions of a reference color and another color are identified from a result of the detection, by which a timing at which an image of each color starts to be formed is adjusted in such a manner that a deviation amount between the relative positions becomes zero.
As an example of the positional deviation correction, a technique discussed in Japanese Patent Application Laid-Open No. 2001-318501 corrects the positional deviation by detecting normal reflected light from a toner image as the positional deviation correction pattern and the intermediate transfer member. In the method that detects normal reflected light, edges of the patch of each color at both ends thereof are identified and the position of the patch is determined by utilizing the fact that a large amount of light is reflected from the intermediate transfer member while a small amount of light is reflected from the toner image. Further, a technique discussed in Japanese Patent Application Laid-Open No. 2012-237904 corrects the positional deviation by detecting diffuse reflected light from a toner image as the positional deviation correction pattern and the intermediate transfer member. In the method that detects diffuse reflected light, edges of the patch of each color at both ends thereof are identified and the position of the patch is located by utilizing the fact that a small amount of light is reflected from the intermediate transfer member while a large amount of light is reflected from the toner image.
However, use of the detection methods discussed in Japanese Patent Application Laid-Open No. 2001-318501 and Japanese Patent Application Laid-Open No. 2012-237904 may result in deterioration in accuracy of the detection of the positional deviation correction pattern depending on a status of the image forming apparatus.
For example, in the method that detects normal reflected light as discussed in Japanese Patent Application Laid-Open No. 2001-318501, the accuracy of the positional deviation correction may be deteriorated due to a change on the intermediate transfer member over time. More specifically, as the change on the intermediate transfer member advances more and more over time, a foreign substance, a scratch, and the like may be more highly likely attached or generated on a surface of the intermediate transfer member. Various kinds of members such as a photosensitive drum, a secondary transfer roller, and a conductive brush are in abutment with the outer surface of the intermediate transfer member. Further, at a secondary transfer portion, a recording material such as paper contacts the intermediate transfer member. A scratch may be generated on the outer surface of the intermediate transfer member due to sliding contact with these members, and a discharge current generated between the member or the recording material and the intermediate transfer member. Further, a foreign substance or the like introduced from the outside of the image forming apparatus may be attached onto the outer surface and inner surface of the intermediate transfer member. If such a scratch is generated or such a foreign substance is attached, the scratch or the foreign substance changes the surface state of the intermediate transfer member. The change in the surface state causes a change in an amount of the normal reflected light, thereby deteriorating the detection accuracy.
FIG. 16 illustrates the change in the detection result when the scratch or the foreign substance exists on the intermediate transfer member. A waveform illustrated in FIG. 16 is an output waveform that the sensor outputs by detecting the normal reflected light, when the scratch or the foreign substance exists near the edge of the patch on an upstream side thereof in a rotational direction of the intermediate transfer member. The illustration of FIG. 16 indicates that the detected waveform expands due to a change in an output value, which is caused by the generation of the scratch or the attachment of the foreign substance. The position of the patch is identified from a midpoint between a rising edge and a falling edge detected according to a comparison with a preset threshold value, based on this result. In the example illustrated in FIG. 16, the position of the patch is detected to be a position offset by a distance corresponding to a detection error Δ from an actual position toward the upstream side in the rotational direction of the intermediate transfer member, and this incorrect detection results in deterioration in the accuracy of the positional deviation correction. Further, if the scratch or the foreign substance exists at a position that does not overlap the position of the patch, an output value generated by the scratch or the foreign substance is incorrectly detected as the patch, and this incorrect detection also results in deterioration in the accuracy of the positional deviation correction.
Further, for example, in the method that detects diffuse reflected light as discussed in Japanese Patent Application Laid-Open No. 2012-237904, the accuracy of the positional deviation correction may be deteriorated due to a change in a toner cartridge over time. The method that detects diffuse reflected light is prone to a large change in the amount of the reflection light when a change occurs in a density of the patch. Therefore, a reduction or unevenness in the toner density of the patch may make the output waveform corresponding to the patch asymmetrical, leading to incorrect detection of the position of the patch.
FIG. 17 illustrates output waveforms of the patch before and after endurance of the toner cartridge. The illustration of FIG. 17 indicates that, after the endurance, the density is reduced and becomes uneven, so that the patch exceeds the threshold value at a smaller portion than before the endurance. As a result, a position offset from the actual position is detected as the position of the patch, and this incorrect detection results in deterioration in the accuracy of the positional deviation correction.